There exists a known seat apparatus for a vehicle, in which a motor, which serves as a driving source, and a decelerating mechanism for decelerating a driving force generated by the motor and transmitting the decelerated driving force to a mobile portion of the seat is combined, in order to adjust an inclination of a front portion of a seat cushion for the vehicle in an up-and-down direction of the seat and in order to slidably adjust a position of an entire seat in a front-rear direction of the vehicle. Generally, a gear decelerating mechanism is adapted as the decelerating mechanism. More specifically, the gear decelerating mechanism is a mechanism for establishing a predetermined deceleration ratio by engaging gears, so that a large torque is obtained in response to the deceleration ratio. In a seat apparatus for adjusting the inclination of the seat, plural deceleration portions are combined in order to output a larger torque to raise the seat.
Disclosed in US2007/0157752 and DE10327103 are rotational speed changing devices for vehicles, which may be adapted as the decelerating mechanism for the power seat driving apparatus for the vehicle. According to the rotational speed changing devices disclosed in US2007/0157752 and DE10327103, a worm and a helical gear are engaged with each other in order to largely decelerate a rotating speed of a motor while generating a torque necessary for operating the seat apparatus. Generally, such decelerating mechanism includes gears and a bearing portion formed at a housing or the like, which are made of metal, in order to enhance durability against a large load applied thereto. Furthermore, because a clearance needs to be ensured within the rotational speed changing device in order to ensure an allowance for assembling and in view of an operation of the rotational speed changing device (in view of meshing of the gears), a backlash may be generated when the rotational speed changing device is operated. Therefore, the metal needs to be used for the bearing portion, the housing and the like, so that the bearing portion and the housing endure a shock generated by the backlash. Furthermore, a noise is likely to be generated when the worm and the helical gear are engaged with each other because of the backlash. As a result, the noise may be transmitted to a compartment of the vehicle. On the other hand, according to a gear box mounting structure disclosed in JPH11-311303A, flange portions, which are provided at an output shaft, are configured so as to contact corresponding brackets, which are provided at a housing, so that a large load generated in an axial direction of an output shaft is also received by the flange portions in order to decrease a backlash. Accordingly, enhancing in accuracy in engagement between the worm and the helical gear while reducing the occurrence of the backlash within the rotational speed changing device is required in order to enhance a reliability of the rotational speed changing device, reduce a manufacturing costs and the like.
Disclosed in U.S. Pat. No. 4,799,403 is a gear adjusting apparatus for a vehicle, in which plural decelerating portions are combined because a sufficient reduction gear ratio is not established by using a single decelerating mechanism. The gear adjusting apparatus for the vehicle disclosed in U.S. Pat. No. 4,799,403 includes a first decelerating portion, which is configured with a worm and a helical gear, and a second decelerating mechanism, which is configured with a planetary gear mechanism. The first decelerating portion and the second decelerating mechanism are connected in series. According to the gear adjusting apparatus for the vehicle disclosed in U.S. Pat. No. 4,799,403, a clearance ensured at the gear adjusting apparatus for the vehicle is decreased and a backlash generated when the worm and the helical gear are engaged with each other is decreased by using the planetary gear mechanism.
In these days, downsizing and cost-reduction of the decelerating mechanism are required. However, the gear and the bearing portion disclosed in US2007/0157752 and DE10327103 are relatively heavy because the gear and the bearing portion are made of the metal. Furthermore, processing of the metal-made gear and the bearing portion used for US2007/0157752 and DE10327103 is inefficient. Therefore, if the gear and the bearing portion are made of resin, the processing efficiency may be improved. However, the accuracy in engagement between the gears needs to be ensured, and furthermore, a mechanism strength of the gear and the bearing portion needs to be ensured. On the other hand, according to the gear adjusting apparatus for the vehicle disclosed in U.S. Pat. No. 4,799,403, the second decelerating portion, which is configured with the planetary gear mechanism, may reduce the backlash, however, a size of the second decelerating mechanism is relatively large. Furthermore, a number of components used for the second decelerating gear is relatively large. Therefore, the manufacturing costs of the gear adjusting apparatus for the vehicle is relatively high. Accordingly, there exists a need to provide a decelerating mechanism, which has a simple configuration, which is light-weighted and which is manufactured with relatively low costs.
A need thus exists to provide a power seat driving apparatus for a vehicle, which is not susceptible to the drawback mentioned above.